A typical DRAM cell includes a transistor and a storage capacitor. In the early DRAM cells, storage capacitors of the planar type were used which require the use of large wafer real estate. In recent years, as the size of IC device is continuously miniaturized when smaller chips are made and more devices are packed into the chips, the circuit density on the chip increases to such an extent that the specific capacitance of a storage capacitor must be increased in order to meet such demand. Since chip real estate is limited, the only feasible way of increasing the specific capacitance of a storage capacitor is to increase it three-dimensionally, i.e., to grow the capacitor cell in the vertical dimension forming a stacked capacitor.
A stacked capacitor is built on top of a transistor thus allowing a smaller cell to be built without losing the specific capacitance. It has become a popular design in modern semiconductor memory devices for saving chip real estate. Other approaches such as the formation of a deep trench capacitor for storing charges vertically requires a deep trench formation and thus encounters various processing difficulties.
In modern memory cells, where smaller dimension and higher specific capacitance are desirable characteristics, a DRAM capacitor can be formed by two layers of a semi-conducting material and one layer of a dielectric material sandwiched thereinbetween. A suitable dielectric material utilized in such a capacitor includes a thin oxide layer or a composite oxide-nitride-oxide layer sandwiched between two semi-conducting layers of polysilicon for forming the capacitor cell. The capacitor is frequently formed over a bit line on the surface of a silicon substrate.
A typical 16-Mb DRAM cell is shown in FIG. 1. The DRAM cell 10 has a stacked capacitor 20 built on top. The formation of the DRAM cell 10 can be accomplished by first using standard CMOS fabrication steps to form the transistor and providing a gate oxide layer (not shown). A word line 12 is then formed by first depositing a polysilicon layer of approximately 2500 .ANG. and then doping the polysilicon with phosphorous. A thick layer of insulating material 16 such as TEOS (tetraethoxy silicate) oxide of approximately 3,000 .ANG. is then deposited on top of the first polysilicon layer. By using standard photomasking processes, the two layers are etched by a plasma etching technique. After LDD implants are made in the silicon substrate, oxide spacers are formed on the polysilicon gate structure by depositing a thick layer of TEOS oxide of approximately 2,000 .ANG. and then etched by a plasma process. Gates 12 and 14 are thus formed and covered by a thick oxide insulating layer 16. A source and drain mask is then applied to carry out an ion implantation process for forming the source and drain regions in the silicon substrate.
In the next fabrication step, photomasking is used to form openings for the cell contact and plasma etching is used to remove any native oxide layer on the silicon substrate. A second polysilicon layer 22 of approximately 3,500 .ANG. is then deposited and patterned by a photomask to form the lower electrode of the stacked capacitor 20. A dielectric layer 24 of a composite film of oxide-nitride-oxide (ONO) is deposited as the dielectric layer for the capacitor. The total thickness of the ONO composite film is approximately 70 .ANG.. The ONO composite film can be formed by using a thin layer of native oxide as the first oxide layer, depositing a thin nitride layer on top and then oxidizing the nitride layer to grow a top oxide layer. To complete the fabrication of the stacked capacitor, a third polysilicon layer 24 of approximately 2,000 .ANG. thick is deposited on top of the dielectric layer and then doped and patterned by a photomask to form an upper electrode. After the formation of the stacked capacitor, peripheral devices can be formed by masking and ion implantation, followed by the formation of a bit line 28 of a polysilicon/metal silicide material. A thick insulating layer 32 of BPSG or SOG is then deposited over the capacitor and reflowed to smooth out the topography and to reduce the step height. Other back-end-processes such as metalization to form metal lines 34 are used to complete the fabrication of the memory device 10.
The stacked capacitor 10 shown in FIG. 1 has been successfully used in 16 Mb DRAM devices. However, as device density increases to 256 Mb or higher, the planar surface required for building this type of conventional stacked capacitors becomes excessive and must be reduced.
Another technique of forming DRAM stack capacitors by using a rugged polysilicon layer as the lower electrode in a capacitor cell is shown in FIG. 2. Referring now to FIG. 2, wherein a semiconductor substrate 40 is shown which has a layer of a non-doped silicate glass 42 deposited on top. After the insulating layer 42 is patterned and etched in a conventional etching process, the substrate area 44 is exposed as the storage node capacitor cell contact. In the next processing step, a layer of polysilicon 46 is deposited and formed. The thickness of the polysilicon layer 46 is in the range between about 400 .ANG. and about 600 .ANG.. To increase the surface area of the polysilicon layer, a rugged surface polysilicon layer 48 is deposited at a relatively low deposition temperature of between about 500.degree. C. and about 600.degree. C. by a chemical vapor deposition technique. The deposition temperature of the rugged polysilicon must be kept low in order to maintain the wave-like surface texture of the rugged polysilicon. The thickness of the rugged polysilicon layer 48 is between about 700 .ANG. and about 1000 .ANG.. In a subsequent process, the polysilicon layer 46 and the rugged polysilicon layer 48 are patterned and etched to form a lower electrode of the capacitor cell. A second insulating layer, preferably of an oxide or an oxide-nitride-oxide insulating layer 50 is deposited by a chernical vapor deposition technique. After the second insulating layer 50 is patterned and etched to form a conformal layer on the capacitor cell, a final layer 52 of polysilicon is deposited by cherrical vapor deposition and formed as the upper electrode in the capacitor cell. Even though a storage capacitor having improved storage capacity can be fabricated by this process, the fabrication process is complicated since a low temperature formation of the rugged polysilicon layer is required. Furthermore, in a capacitor cell that utilizes rugged polysilicon, the device must not be subjected in a down-stream fabrication step to at a temperature of higher than 600.degree. C. which would render the wave-like textured surface of the rugged polysilicon smooth and loses its increased storage capacity.
It is therefore an object of the present invention to provide a method for forming a DRAM capacitor that has improved storage capacity and not the drawbacks or shortcomings of the conventional capacitors.
It is another object of the present invention to provide a method for forming a DRAM capacitor that has improved storage capacity but does not require the formation of low temperature rugged polysilicon as an electrode of the capacitor.
It is a further object of the present invention to provide a method for forring a DRAM capacitor that has improved storage capacity by depositing a metal layer on a polysilicon forming a metal silicide and then heat treating the metal suicide at a temperature sufficient to form an island structure.
It is another further object of the present invention to provide a method for forming a DRAM capacitor by first forming a metal silicide layer having an island structure on a polysilicon layer and then isotropically etching the polysilicon layer into the same island structure for use as a lower electrode of the capacitor.
It is yet another object of the present invention to provide a method for forming a DRAM capacitor that has improved storage capacity by the formation of a metal suicide layer that is capable of forming an island structure after a rapid thermal process.
It is another object of the present invention to provide a method for forming a DRAM capacitor that has improved storage capacity by using a metal silicide layer formed on a polysilicon layer that develops an island structure after heat treating to a temperature higher than 500.degree. C.
It is still another further object of the present invention to form a DRAM capacitor that has improved storage capacity by using a molybdenum suicide layer capable of forming an island structure after exposure to 600.degree. C. and then isotropically etching the underlying polysilicon layer into the same island structure for use as the lower electrode.
It is yet another further object of the present invention to provide a DRAM capacitor that has improved storage capacity by using a lower electrode formed of polysilicon and etched into an island structure such that its surface area is increased to improve its storage capacity.